Another involved offering two plates of food and measuring how quickly the dogs would go to the one with the bigger portion.

They said that because "confounding" factors such as drinking, smoking and different socio-economic backgrounds did not apply, it was easier to measure differences in intelligence and links between longevity and intelligence than with humans.

Dogs also develop dementia in similar ways to their human masters, the researchers said in a research paper published in Intelligence, meaning that the findings could be comparable to human beings.

Creatures both big and small made the list of the top 10 most intelligent organisms on the planet, proving that bigger isn't always better when it comes to brains.
The new list, created by neuropsychiatrist

, includes animals that have been widely known for their smarts, as well as some surprising entries.
Boston-based Lieff, who is a past president of the American Association for Geriatric Psychiatry, specializes in the interface between psychiatry, neurology, and medicine. Such interdisciplinary research is leading to a better understanding on how human and other mammal brains age, which may shed light on treatments for common disorders, such as dementia.
Included on the list are bees, which Lieff said use symbolism and abstract concepts to solve problems in their daily lives.
"They have a kaleidoscopic memory of every flower for miles, and learn from wise elders where the best flowers are," he told Discovery News. "Bees self medicate in their hives in different ways, and engineer the very complex honeycomb structures that are the most efficient possible for honey storage."

Do not let their squishy tentacles fool you. Octopuses are extremely intelligent, according to Lieff, who explained that they spread cultural information, mimic others and communicate using colors, patterns and flashing.
"They have advanced spatial learning capacity, navigational abilities and use creative predatory techniques," he added. "They manipulate objects as well as the human hand does."

Elephants have highly evolved social capabilities and often show wise, compassionate and loving behaviors, Lieff said.
Elephants have amazing memories and can remember friends and enemies for a half century or more, depending on their health and lifetime. (Elephants in the wild can live to about 60, and the oldest known elephant on record -- in captivity -- reached the age of 86.)
"In captivity, elephants have become excellent artists," Lieff said. "Elephant communication is elaborate involving many different vocalizations, and they 'speak' to family five miles away. Elephants are extremely collaborative, consoling and cooperative, and deeply mourn their dead."

Ants are the ultimate team players, but they display great individual intelligence as well, according to Lieff. The insects can navigate long distances with ease, remembering their lengthy routes.
"Ants care for their family similar to many mammals and show altruism for comrades," Lieff said, mentioning that ants are the second society on earth (after termites) to develop agriculture. Human-established agriculture evolved much later.

Dolphins continue to astound researchers. These marine mammals in captivity can remember the communications of their fellow tank mates for at least 20 years, according to Lieff.
"They also immediately notice themselves in a mirror, and are very creative in inventing new ways to fish," he continued. "They wear sponges on their noses to protect themselves near rocks. They have advanced social relations with humans, such as herding large schools of fish toward fishermen wading in the water with nets. When the fish are about to come in contact with the nets, the dolphins signal the fishermen by slapping the water."

Birds are among the smartest of animals, with two groups making this particular list. The first are crows.
"Crows are aware of themselves and are able to use counting and analogies," Lieff said. "They can solve higher-order, relational-matching tasks spontaneously. They use tools better than most (non-human) primates, molding wire into a hook and using three different tools for one task."
He added that studies reveal crows understand the Archimedes principle, which holds that the upward buoyant force exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
This knowledge allows crows to manipulate water levels to access food.

Curious and playful, cockatoos are also incredibly smart.
Lieff explained that cockatoos use multi-step techniques to solve problems, such as figuring out how a complex cage lock works. For example, Lieff said one clever cockatoo figured out a lock that required removal of a screw, followed by removal of a bolt, then turning a wheel 90 degrees before finally shifting a latch sideways. Other cockatoos that watched this happen quickly repeated the successful maneuver.
A quick search at YouTube also reveals that cockatoos "completely disprove the notion that animals can't dance," Lieff said.

Lizards do not get a lot of recognition for their intelligence, but Lieff believes it is time they did. Anole lizards, in particular, continue to impress researchers with their cognitive skills.
Anoles demonstrate counting, advanced learning and problem solving, he said, adding that their memories are extraordinary.
In captivity, these lizards can "invent techniques that they don't use in the wild. They can use multiple different strategies (to solve puzzles) and can unlearn incorrect approaches, rapidly reversing course."

Dogs are so loyal to humans that they often don't get credit for their own intelligence.
"From a purely cognitive vantage point, dogs have learned up to a thousand different words," Lieff said.
He added that "service dogs demonstrate creativity and high intelligence" in saving others. Canines can also accurately read human emotions.

Rounding out the list are whales, which demonstrate elaborate communication techniques and cultural exchange.
"They work together for creative fishing techniques, with each whale in a designated role," Lieff said.
For example, he explained three whales sometimes line up in a row while beating their tails together. This creates waves that can knock a seal off an ice perch.
Whales also swim beneath a school of fish in circles, blowing air bubble "nets," which trap the fish. This requires a lot of team organization, as some whales do the diving while others make calls in order to herd the fish. Such teamwork is passed on to other generations.

Missing from this Top 10 list are humans and other primates, which clearly excel at social communication, cooperation, tool making and much more.
But can a species measure with complete accuracy its own intelligence compared to that of other animals? It's near impossible, since that judgment can only be based on the particular animal's values, brain structure and way of thinking.